In a liquid crystal display device, a classification based on an operating mode for liquid crystal molecules includes modes such as PC (phase change), TN (twisted nematic), STN (super twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), VA (vertical alignment), FFS (fringe field switching) and FPA (field-induced photo-reactive alignment). A classification based on a driving mode in the device includes PM (passive matrix) and AM (active matrix). The PM is classified into static, multiplex and so forth, and the AM is classified into TFT (thin film transistor), MIM (metal-insulator-metal) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type depending on the production process. A classification based on a light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both natural light and a backlight.
The liquid crystal display device comprises a liquid crystal composition having a nematic phase. This composition has suitable characteristics. An AM device having good characteristics can be obtained by an improvement of the characteristics of this composition. Table 1 below summarizes the relationship between these two characteristics. The characteristics of the composition will be further explained on the basis of a commercially available AM device. The temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or higher and a desirable minimum temperature of the nematic phase is approximately −10° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying moving images on the device. Response time that is one millisecond shorter than that of the other devices is desirable. Thus a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable. The elastic constant of the composition relates to the contrast ratio of the device. A large elastic constant of the composition is desirable for increasing the contrast ratio of the device.
TABLE 1Characteristics of Compositions and AM DevicesCharacteristics ofCharacteristics ofNo.CompositionsAM Devices1wide temperature range ofwide temperature range in whichnematic phasedevice can be used2small viscosity 1)short response time3suitable optical anisotropylarge contrast ratio4large positive or negativelow threshold voltage and low powerdielectric anisotropyconsumption, large contrast ratio5large specific resistancelarge voltage holding ratio andlarge contrast ratio6high stability to ultravioletlong service lifelight and heat7large elastic constantlarge contrast ratio and shortresponse time1) A composition can be injected into a liquid crystal display device in a shorter period of time.
The optical anisotropy of the composition relates to the contrast ratio of the device. A large optical anisotropy or a small optical anisotropy, namely a suitable optical anisotropy, is necessary depending on the mode of the device. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on the type of operating mode. A suitable value is approximately 0.45 micrometer for a device having a mode such as TN. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, low power consumption and a large contrast ratio of the device. A large dielectric anisotropy is thus desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. It is thus desirable that a composition should have a large specific resistance at a temperature close to the maximum temperature of a nematic phase as well as at room temperature in the initial stages. It is desirable that a composition should have a large specific resistance at a temperature close to the maximum temperature of a nematic phase as well as at room temperature, after it has been used for a long time. The stability of the composition to ultraviolet light and heat relates to the service life of the device. The device has a long service life when the stability is high. These kinds of characteristics are desirable for an AM device used for a liquid crystal projector, a liquid crystal television and so forth.
A composition having positive dielectric anisotropy is used for an AM device having a TN mode. A composition having negative dielectric anisotropy is used for an AM device having a VA mode. A composition having positive or negative dielectric anisotropy is used for an AM device having an IPS mode or an FFS mode. A composition having positive or negative dielectric anisotropy is used for an AM device with a PSA (polymer sustained alignment) mode. Examples of the liquid crystal composition having positive dielectric anisotropy are disclosed in the following patent document No. 1 or No. 2.